The subject invention generally relates to power switching circuits utilizing semiconductors with inductive loads and, more particularly, to improvements in snubbing techniques for power diodes used to clamp inductive voltages.
In applications such as, for example, a switching power supply, a switch regulates current through an inductive load by opening and closing at a controlled rate. The switch may be mechanical or solid-state, such as a transistor or thyristor. During the time period when the switch is non-conductive, current continues to flow through the inductive load by virtue of a current path normally established by a free-wheeling or clamp diode connected across the load. The diode provides a current path and serves to limit or clamp the inductive voltage developed by the load.
When the switch is gated into conduction, with load current in the clamp diode, current is transferred from the diode to the switch at a rate which is limited only by the source voltage and the circuit inductance until the peak reverse current of the diode is reached. At this point, the diode tends to snap off at even higher rates of change of current, generating high voltage transients from the energy stored in the stray inductance of the free-wheeling current path. With fast turn-off diodes, the voltage transients are often oscillatory due to the stray inductance ringing with the diode capacitance. In power circuits, without snubbing, these transients can radiate considerable electromagnetic interference (EMI) and can also exceed the reverse blocking voltage of the diode, causing self-destruction.
The recovery characteristics of a diode are depicted by the solid line on the graph of FIG. 1. The rate of change of current during commutation is a function of circuit parameters and, more specifically, of the applied voltage and the total series inductance according to the following formula: (di/dt)=-(E/L), where (di/dt) is rate of change of current, E represents the magnitude of applied voltage and L represents total series inductance.
Before complete turn-off is attained, the diode current must reverse in order to sweep out the stored charge in the device. The peak reverse recovery current (Irr) is directly proportional to the rate of change of current since the area under the negative portion of the graph remains nearly constant for a given charge. In high speed switching circuits, the peak reverse currents can become excessively high and are generally snubbed by various circuit modifications.
One technique for reducing these currents is to reduce the rate of change of current as the current level approaches zero. This can be achieved by connecting a saturable reactor in series with the diode. The reactor comes out of magnetic saturation at low current levels, thereby inserting a larger series inductance, which results in the modification of the current waveform to that illustrated by the dotted line in FIG. 1. The schematic diagram of FIG. 2 is a prior art switching circuit utilizing this method. A variety of devices can be used to perform the switching function of switch 12, but commonly a bipolar power transistor would be used. The switch 12 is connected in a series circuit between a reactive load 11 and a power source 18. A control circuit (not shown) supplies control signals to regulate the on and off time intervals of switch 12 in a manner well known in the art. When switch 12 is closed, current flows from the power source 18 through the series connection of the reactive load 11 and the controlling switch 12. When switch 12 opens, the energy stored in the reactive portion of the load slowly discharges through a clamp diode 13 and a saturable reactor 14 which are serially connected across the load 11. When switch 12 recloses, load current is then commutated from the clamp diode 13 to the switch 12. In addition to assuming the load current, switch 12 also passes the reverse recovery current required to turn off clamp diode 13. Just prior to completion of this commutation, reactor 14 comes out of saturation, inserting more inductance in series with the clamp diode 13 which lowers the peak recovery currents required to turn off the diode 13. This feature allows a lower current rating for switch 12.
A snubber circuit consisting of the parallel connected combination of a diode 15 and a capacitor 16 and a series connected resistor 17 is shown connected across the switch 12 and is sometimes used to absorb some of the power losses that occur each time switch 12 turns off. The shunt snubber is not mandatory if switch 12 can absorb the switching power losses.
The major disadvantage of this technique, i.e., utilizing a simple saturating reactor, is that reactor 14 is not saturated at the moment that switch 12 turns off and, therefore, presents a high inductive impedance in the clamp diode current path. To effect a commutation of load current from switch 12 to diode 13, the voltage at the junction of switch 12 and load 11 must increase to a value which is sufficient to overcome the inductance of the unsaturated reactor 14. This voltage must exceed that of the power source in proportion to the inductance presented by reactor 14 which requires an increased voltage rating for switch 12.